We propose to investigate the mechanisms of gene induction by ionizing radiation that may have novel applications in radiotherapy. Data from our laboratory suggest that genetic constructs which are relatively specific for radiation-induction can be engineered to amplify gene transcription. We will construct recombinant DNA plasmids encoding cytotoxic proteins which will be transcribed following exposure to x-rays. We will insert x-ray inducible DNA sequences obtained from deletion analysis of the c-jun promoter, upstream of the transcription start site of a toxin gene. The specificity of toxin induction will be accomplished by constructing a plasmid encoding a radiation-activated hybrid transcription factor consisting of a yeast DNA-binding peptide (Gal4) and a mammalian transcriptional activator peptide (Jun). Following radiation-activation of the hybrid transcription factor Gal-Jun, the activated protein will bind the yeast GAL4 promoter on a second plasmid encoding a cytotoxic and/or radiosensitizing protein. We will investigate whether our proposed strategy can produce a biologically active protein with cell killing or radiosensitizing properties. We will select tumor necrosis factor alpha for our initial studies because this protein has been demonstrated to be a cytotoxin and/or a radiosensitizer in tumor cells studied in our laboratory. Our studies will characterize a prototype radiation-inducible plasmid encoding a cytotoxic protein and provide a foundation for the development of a genetic radiotherapy.